1. Field of the Invention
The invention relates to a semiconductor light emitting device and a method of manufacturing such a device. More specifically, it relates to a semiconductor light emitting device that emits light of an arbitrary color, by using additive color mixing to combine the light emitted from a semiconductor light emitting element with the wavelength converted light emitted from a phosphor that has been excited by the light from the semiconductor light emitting element, as well as a method of manufacturing such a device.
2. Description of the Related Art
Producing a white light emitting device using a light emitting diode (LED) chip that emits light with a sharp spectral distribution as the light source can be achieved by employing additive color mixing of the light emitted from the LED chip, and wavelength converted light emitted from a phosphor that has been excited by the light emitted from the LED chip.
For example, if the light emitted from the LED chip is blue light; then a phosphor is used that is excited by blue light and causes a wavelength conversion to yellow light, which is the complementary color of blue. Additive color mixing of the blue light emitted from the LED chip with the wavelength converted yellow light emitted from the phosphor, which has undergone excitation under the irradiation of the blue light from the LED chip, enables the production of a white light.
Alternatively, if the light emitted from the LED chip is blue light, then a mixture of two phosphors that are excited by blue light and cause wavelength conversions to green light and red light respectively can also be used. In this case, additive color mixing of the blue light emitted from the LED chip, and the wavelength converted green light and red light emitted from the two phosphors, which have undergone excitation under the irradiation of the blue light from the LED chip, enables the production of a white light.
Furthermore, if the light emitted from the LED chip is ultraviolet light, then a mixture of three phosphors that are excited by ultraviolet light and cause wavelength conversions to blue light, green light, and red light respectively can be used. In this case, additive color mixing of the wavelength converted blue light, green light, and red light emitted from the three phosphors, which have undergone excitation under the irradiation of the ultraviolet light from the LED chip, enables the production of a white light.
In addition, light of colors other than white can also be obtained by suitable combinations of colored light emitted from an LED chip, and a phosphor that functions as a wavelength conversion material.
FIG. 1 shows an example of a related art light emitting device in which the light emitted from the light source excites a phosphor and causes a wavelength conversion, resulting in the emission of light of a different color from that of the light source (see Japanese Patent Laid-open Publication No. 2002-151743). In this example, a cup (concave portion) 52 for housing a light emitting element (LED chip) 51 is formed in the middle of a casing 55 that incorporates a substrate. The light emitting element 51 is positioned in the bottom portion of the cup 52 of the casing 55, and is electrically connected via bonding wire 56. The cup 52 is filled with a resin 54, which contains a dispersed phosphor 53 that functions as a wavelength conversion material. A lid is then placed across the top surface (the open portion of the cup 52) of the casing 55, and the casing is inverted, so that the phosphor 53, which has a larger specific gravity than the resin 54, sinks through the resin and accumulates near the open portion of the cup 52. With the phosphor 53 in this state of uneven distribution near the open portion, the resin 54 is heat cured, thereby yielding a light emitting device in which the density of the phosphor 53 is higher near the open portion of the cup 52 than in the lower portion.
Related art light emitting devices such as that shown in FIG. 2 have also been proposed as alternative means for use as a lighting device (for example, see Japanese Patent Laid-open Publication No. 2003-234511). In this example, a cup 63 of a similar structure to that described above is filled with a first light transmitting resin 64 to a level equivalent to between 60 and 70% of the cup capacity, and this first resin is then heat cured. Subsequently, a second light transmitting resin 66, which contains a dispersed phosphor 65 that functions as a wavelength conversion material, is then injected on top of the first resin, in a quantity equivalent to between 50 and 60% of the cup capacity. The cup is then inverted and heat cured, so that the second light transmitting resin 66 bulges out in a convex shape around the outer edges of the cup 63, while the phosphor 65 dispersed within the second light transmitting resin 66 sinks through the resin and accumulates near the convex bulge at the cup opening. As a result, a light emitting device is formed in which the phosphor 65 is distributed at high density near the convex-shaped surface of the resin.
In the former of the conventional light emitting devices described above, when the lid is placed across the top surface of the casing, and the resin inside the cup is heat cured with the casing inverted, the top surface of the casing and the lid must contact with no gaps around the entire top surface. If even a small gap exists, then the resin can leak out of the cup through that gap, resulting in a defective product.
Particularly, when a plurality of cups are formed within a large casing to enable large volume batch production, ensuring the high level of surface precision necessary to ensure a tight contact between the top surface of the cups and the lid is extremely difficult. Furthermore, the heat of the resin heat curing process can cause deformations such as expansion or warping of the casing and/or the lid, thereby destroying the contact between the two members, and deterioration in the production yield is thus unavoidable. Even if prevention of such problems were possible, the associated costs would be considerable.
In addition, in order to increase the quantity of light emitted, the size of the light emitting element can be increased, and a larger current can be passed through the element, but there are practical limits to the size of the package. This means the size of the cup in which the light emitting element is located is also limited, which causes an increase in the proportion of the internal capacity of the cup occupied by the light emitting element, when compared with conventionally sized light emitting devices of a similar type. In other words, there is a reduction in the free capacity within the cup, calculated by subtracting the volume of the light emitting element from the total internal capacity of the cup.
As a result, both the distance between the side surfaces of the light emitting element and the inner peripheral surface of the cup, and the distance between the upper surface of the light emitting element and the top surface of the resin used to fill the cup are reduced. This means that the quantity of resin that exists between the upper surface of the light emitting element and the top surface of the resin is significantly less than the quantity of resin that exists between the side surfaces of the light emitting element and the inner peripheral surface of the cup. This can also be said of the quantity of phosphor dispersed within the resin prior to curing.
If a lid is placed across the top surface of this type of casing, and the casing is then inverted and the resin subjected to heat curing, then as described above, the quantity of phosphor present in the resin between the upper surface of the light emitting element and the top surface of the resin is significantly less than the quantity of phosphor present in the resin between the side surfaces of the light emitting element and the inner peripheral surface of the cup. Consequently, the quantity of phosphor that sinks into the region positioned above the light emitting element is less than the quantity that sinks into the surrounding region, meaning formation of a uniform phosphor layer is difficult.
As a result, the light emitted from the light emitting element causes the excitation of varying quantities (densities) of the phosphor depending on where it reaches the high density phosphor layer, meaning the light emission displays considerable color irregularity. Color irregularity is governed by strict regulations (specifications), particularly for white light LEDs, meaning the above problem causes a significant reduction in the yield of product.
In contrast, in the latter of the conventional light emitting devices described above, surface tension causes the first light transmitting resin to climb up the outside edges of the cup. The second light transmitting resin with the phosphor dispersed therein is then used to form a convex lens-shaped layer on top of this first resin, so that a high density phosphor layer is provided near the surface of this convex lens-shaped bulge. During the formation of this layer, the quantity of phosphor near the edges of the second light transmitting layer is significantly less than that within the central bulge portion. Furthermore, the second light transmitting resin may have trouble reaching the top of the first light transmitting resin around the outside edges of the cup, meaning a phosphor layer may not form at these outside edges.
In the ideal situation, a light emitting device should be constructed so that additive color mixing of the light emitted from the light emitting element with the light that has been emitted from the light emitting element and which has then undergone wavelength conversion within the phosphor layer, generates a white light with minimal color irregularity in substantially all directions. However, if light does not pass through the phosphor layer within some regions, but is rather emitted directly from the light emitting element, then within those regions, the emitted light is not a product of additive color mixing, but is solely the light emitted from the light emitting element having its original color.
In such a case, if the light emitted from the light emitting element is blue light with a peak wavelength of approximately 450 to 470 nm, then light which is emitted from the light emitting element and which passes through a region in which the phosphor layer exists is radiated from the light emitting device as white light (W). In contrast, light which is emitted from the light emitting element and passes through a region in which the phosphor layer does not exist is radiated from the light emitting device as blue light (B). This means the white LED product actually emits a mixture of both white and blue light, which is undesirable.
Furthermore, suppose the light emitted from the light emitting element is from the short wavelength region with a peak wavelength of no more than 400 nm. Then, if this ultraviolet light is radiated directly from the light emitting device, and if it enters a person's eyes directly, it may present potential dangers.